The present invention relates to an improved wire pull back delivery system. More specifically, the invention relates to a wire pull-back stent delivery system which utilizes a shape memory contraction member to retract the retractable outer sheath and deploy a medical implant for a minimally invasive application, such as an endovascular stent graft, vena cava filter, aneurysm repair particles, self-expanding stent, balloon expandable stent, or the like.
Delivery systems for deploying medical implants, such as an endovascular stent graft, vena cava filter, self-expanding stent, balloon expandable stent or the like, are a highly developed and well known field of medical technology. These medical devices have many well known uses and applications. In particular, a stent is a prosthesis which is generally tubular and which is expanded radially in a vessel or lumen to maintain its patency. Stents are widely used in body vessels, body canals, ducts or other body lumens. Balloon expandable stents are mounted on a balloon which when expanded delivers the stent, exerting radial force on the constricted portion of the body lumen to re-establish patency. A self-expanding stent is a stent which expands from a compressed delivery position to its original diameter when released from the delivery device, exerting radial force on the constricted portion of the body lumen to re-establish patency. One common self-expanding stent is manufactured of Nitinol, a nickel-titanium shape memory alloy, which can be formed and annealed, deformed at a low temperature, and recalled to its original shape with heating, such as when deployed at body temperature in the body. A common material for balloon expandable stents is stainless steel.
Wire pull-back stent delivery systems commonly assigned with this application include U.S. Pat. No. 5,571,135, U.S. Ser. No. 08/753,641 filed Sep. 27, 1996 and U.S. Pat. No. 5,733,267, the entire contents of which are hereby incorporated by reference. Another wire pull-back stent delivery system is shown in U.S. Pat. No. 5,360,401. One important factor in delivering the stent is a controlled precise retraction of the retractable outer sheath. What is needed is a wire pull-back stent delivery system which provides for a controlled and precise retraction of the retractable outer sheath and enables the physician to accurately determine proper positioning of the stent.
The inventive stent delivery system includes a catheter having a retractable outer sheath near its distal end. A shape memory contraction member having a memorized contracted shape is connected to the retractable outer sheath. A heat generating device connected to the shape memory contraction member causes the shape memory contraction member to heat up to its transition temperature and assume its contracted position, retracting the retractable outer sheath.
Another embodiment utilizes 2 springs, a xe2x80x9cnormalxe2x80x9d spring and a shape memory alloy (SMA) spring, the two springs selected and designed so that the xe2x80x9cnormalxe2x80x9d spring has an expansion force which is less than SMA spring when the SMA spring is austenitic, but greater than the SMA spring when the SMA spring is martensitic.
Yet another embodiment utilizes a shape memory latch which in its martensitic state abuts a stop to prevent a spring from moving the sheath proximally, but in its austenitic state releases the stop, allowing the spring to retract the sheath to release the stent for deployment.